1. Field of the Invention
The invention relates to a device and a method for producing at least one silicon carbide (SiC) single crystal. A device and a method of this type are known, for example, from International Patent Disclosure WO 94/23096 A1.
International Patent Disclosure WO 94/23096 A1 discloses a device and a method for producing an SiC single crystal which use the sublimation of SiC in solid form, for example of industrial-grade SiC in powder form, and the deposition of the SiC in a gas phase formed as a result of the sublimation on a single-crystalline SiC seed crystal. A reaction vessel in crucible form is used, which contains a storage region and a reaction region which are connected to one another by a gas duct. Alternatively, an additional homogenization region may be connected between the storage region and the reaction region, which homogenization region is likewise in communication with the storage region and the reaction region, in each case via a gas duct. The storage region contains the solid SiC, whereas the single-crystalline SiC seed crystal on which the SiC single crystal grows is disposed in the reaction region. In the document, embodiments in which a plurality of SiC single crystals are deposited on in each case associated SiC seed crystals are also described. In addition, various configurations of the storage regions are disclosed. Outside the reaction vessel there is a heater device which in particular may also be of a multi-part structure in accordance with the division of the reaction vessel into the storage region and the reaction region. The heater device heats the stock of solid SiC in the storage region to a temperature of from 2000xc2x0 C. to 2500xc2x0 C. As a result, the solid SiC is sublimed. The gas mixture which is formed in the process primarily contains the components Si (silicon), Si2C, SiC2 and SiC. The gas mixture is also referred to below as xe2x80x9cSiC in the gas phasexe2x80x9d.
As a result of a temperature gradient being established between the stock of solid SiC and the SiC seed crystal or the SiC single crystal which has already grown, the sublimed gas mixture is conveyed from the storage region into the reaction region, in particular to the SiC seed crystal. In this case, the flow of the SiC in the gas phase is set in terms of its conveying rate and also its direction by the geometry of the gas duct.
The individual constituents of the reaction vessel preferably are formed of a high-purity electrographite. This is isostatically pressed graphite. These types of graphite are commercially available in various densities. They differ in terms of their relative density and different porosity. Even very highly pressed graphites still have a pore volume of at least 8 to 12%. The residual porosity is of importance for the silicon carbon growth, since the gases which are present during the silicon carbide growth, in particular the silicon-containing gases, penetrate into the pores, where they react with the graphite.
The article titled xe2x80x9cFormation of Macrodefects in SiCxe2x80x9d, by R. A. Stein, Physica B, Vol. 185, 1993, pages 211 to 216, describes a phenomenon which relates to the reaction of solid SiC with the carbon in the graphite forming the vessel material. According to this, small pores or dislocations in the region of an interface between a base material made from graphite and an SiC seed crystal disposed thereon form the starting point initially for the formation and then also the subsequent growth of cavities in the SIC seed crystal. Under the conditions that prevail in the reaction vessel, these cavities extend beyond the seed crystal and also into the SiC single crystal to be produced. These cavities lead to a reduced quality of the SiC single crystal being produced.
As is known from International Patent Disclosure WO 94/23096 A1, material is conveyed from the stock of solid SiC to the SiC seed crystal as a result of a temperature gradient being established and a heat flux which forms as a result. When controlling heat fluxes in the crucible using parts or inserts made from graphite, the difficulties that have already been mentioned above occur again, on account of the reaction between the SiC in the gas phase and the graphite.
Published, Soviet Patent Application SU 882247 A1 discloses the use of tantalum as a suitable material for the crucible or at least for an insert inside the crucible. However, tantalum also reacts with SiC in the gas phase. In particular, carbides are formed, so that the dimensions of the device containing the tantalum change. For example, if tantalum thicknesses of several millimeters are provided, this may lead to mechanical stresses in the crucible.
U.S. Pat. No. 5,667,587 discloses a crucible for sublimation growth of an SiC single crystal, the inner walls of which crucible are coated with a thermally anisotropic coating. In particular, the coating is formed of pyrolitic graphite. The thermal anisotropy of the pyrolitic graphite in this case serves to control heat fluxes inside the crucible as referred to above. However, since the coating, just like the gas duct disclosed in International Patent Disclosure WO 94/23096 A1, is formed of a graphite material, it also has undesirable reactions with the SiC in the gas phase occur. In this context, it is irrelevant whether electrographite or pyrolitic graphite is used.
It is accordingly an object of the invention to provide a device and a method for producing at least one SiC single crystal, that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which allow heat fluxes in the crucible to be controlled and, at the same time, avoid the undesirable reactions of the materials used in the prior art with solid SiC or also with SiC in the gas phase.
With the foregoing and other objects in view there is provided, in accordance with the invention, a device for producing at least one silicon carbide (SiC) single crystal. The device contains a crucible having at least one storage region for holding a stock of solid SiC and at least one crystal region for holding in each case one SiC seed crystal on which the SiC single crystal grows. A heater device is disposed outside the crucible, and at least one insert made from glassy carbon is disposed in the crucible.
With the foregoing and other objects in view there is also provided, in accordance with the invention, a method for producing at least one silicon carbide (SiC) single crystal. The method includes the steps of:
a) introducing a stock of solid SiC into at least one storage region of a crucible;
b) introducing at least one SiC seed crystal into the crucible;
c) providing at least one insert made from glassy carbon in the crucible for controlling heat flux;
d) heating the solid SiC such that the solid SiC is sublimed and results in SiC in a gas phase being generated; and
e) conveying the SiC in the gas phase to the at least one SiC seed crystal, on which it grows forming the SiC single crystal.
The invention is based on the recognition that glassy carbon, on account of its excellent properties, is eminently suitable for use in a crucible that is used to produce SiC single crystals. Glassy carbon is an amorphous, isotropic material which has a melting point which lies considerably above the temperature of up to 2500xc2x0 C. which is customarily employed during the production of SiC single crystals. Since, moreover, glassy carbon has a higher density and, with a pore volume of virtually 0%, a significantly lower porosity than all types of graphite, the glassy carbon also presents a considerably reduced tendency to react with both solid SiC and with SiC in the gas phase compared to graphite. Moreover, the thermal conductivity of glassy carbon is lower than that of graphite by a factor of approximately 10. For this reason, glassy carbon is a better thermal insulator than graphite. Therefore, heat fluxes in the crucible can be guided in specific directions by inserts made from glassy carbon.
On account of its high thermal insulating properties, glassy carbon fulfills the requirements for controlling heat fluxes inside the crucible. Furthermore, the tendency of glassy carbon to react with SiC is considerably lower, on account of its high density, than that of other materials used according to the prior art, such as for example different types of graphite.
A first preferred configuration provides for a specific control or guidance of a first heat flux. The first heat flux leads to the SiC in the gas phase being guided in a controlled manner onto a crystallization front at the SiC seed crystal or at the SiC single crystal which has already grown on. The first heat flux is now controlled in particular in such a way that, at least at the location of the crystallization front, i.e. at the point at which the crystal growth on the SiC seed crystal or the SiC single crystal which has already grown on is currently taking place, it has a uniform orientation (=parallel flux vectors) over its entire cross section. This property of the first heat flux is preferably already established within a zone which precedes the crystallization front. As a result, both a uniform temperature (=isothermal plane) and a uniform concentration of the SiC in the gas phase (=plane of identical material concentration) are established at the crystallization front itself. This has a beneficial effect with regard to homogeneous and flawless crystal growth.
In another advantageous embodiment, a hollow cylindrical gas duct made from glassy carbon is situated in the interior of the crucible and specifically between the storage region and the first crucible wall. The SiC seed crystal on which the SiC single crystal grows is positioned at an end of this gas duct that is remote from the storage region. On account of a temperature gradient which has been established, SiC in the gas phase which has sublimed out of the stock moves to an end of the gas duct which faces the storage region. A first heat flux forms through the gas duct, leading to the SiC in the gas phase being passed in a controlled manner to the crystallization front at the SiC seed crystal or at the SiC single crystal which has already grown on. On account of the good thermal insulation properties of the glassy carbon used for the gas duct, the focusing of the first heat flux and therefore also the conveying of the SiC in the gas phase take place particularly effectively. There are only slight heat losses through walls of the gas duct.
An embodiment in which a principal direction of the heat flux runs virtually parallel to a first center axis associated with the SiC single crystal is advantageous. The result is a particularly favorable planar or slightly convex growth phase boundary at the growing SiC single crystal. In this context, a virtually parallel orientation is considered to be present provided that the principal direction of the first heat flux and the first center axis associated with the SiC single crystal include an angle of less than 10xc2x0. Since the first heat flux is decisively controlled by the gas duct, its principal direction has practically the same orientation as a second center axis associated with the gas duct.
In another preferred variant of the device, the control of the first heat flux is improved as a result of a wall of the gas duct running along the second center axis being configured so that its wall thickness is not constant. The claimed improvements are achieved by a corresponding variation in the wall thickness along the second center axis. This is achieved in particular if the wall thickness increases constantly starting from the SiC seed crystal toward the storage region.
Preferred a wall thickness of the wall of the gas duct is between 0.1 and 5 mm. Glassy carbon can be produced to these thicknesses without problems.
In addition to the discussed embodiments with control of a first heat flux inside the crucible, it is also possible to provide embodiments in which heat fluxes between the interior and exterior of the crucible are controlled by the use of an insert made from glassy carbon.
An advantageous embodiment results if a first plate made from glassy carbon is disposed on a side of the SiC seed crystal which is remote from the SiC single crystal. In this case, the first plate fulfills a plurality of functions simultaneously. On account of the flexural strength of glassy carbon being three to four times higher than that of graphite, the first plate can be made significantly thinner than a corresponding plate of graphite. The result is easier dissipation of heat from the SiC seed crystal. To maintain the temperature gradient between the stock and the SiC seed crystal, it is necessary for heat to be dissipated at the SiC seed crystal. This takes place via the first plate.
A preferred thickness range for the first plate is between 0.1 and 2 mm. A plate thickness of 0.5 mm is particularly preferred.
A second very significant function of the first plate made from glassy carbon consists in avoiding undesirable reactions between the SiC seed crystal and the base. When the SiC seed crystal is applied to a base made from graphite, for example directly to a first crucible wall on an upper inner side of the crucible, the silicon carbide of the SiC seed crystal reacts with the carbon base. As a result, cavities are formed which also propagate inside the growing SiC single crystal, thus reducing the quality of the SiC single crystal produced. SiC single crystals of this type can no longer be used for all applications.
It has now been found that the application of the SiC seed crystal to a base of glassy carbon leads to a considerably improved quality of the SiC single crystal produced. The reason for this lies in the materials properties of glassy carbon. Glassy carbon is significantly less likely to react with the SiC of the SiC seed crystal. Since, therefore, there are no cavities formed at the interface between the SiC seed crystal and the first plate made from glassy carbon, the density of cavities in the SiC single crystal produced is also considerably reduced.
In addition, it is advantageous if the first plate has a polished surface facing toward the SiC seed crystal.
In another embodiment, the storage region is thermally insulated by a second plate of glassy carbon, which is situated at the bottom of the storage region. In particular, the second plate may simply be laid on top of a second crucible wall, which delimits the storage region at the bottom. The second plate made from glassy carbon therefore prevents heat loss in the storage region. In this advantageous embodiment, a third heat flux, which without the second plate would dissipate heat from the storage region and the crucible, is returned to the storage region. In this way, the thermal energy in the storage region is maintained and contributes here to the sublimation of the SiC.
In accordance with an added feature of the invention, there is the step of disposing the SiC seed crystal on a wall of the crucible which is spaced apart and lies opposite from the storage region.
In accordance with an additional feature of the invention, there is the step of controlling the heat flux, which conveys the SiC in the gas phase to the SiC seed crystal, in such a way that an isothermal plane and a plane of uniform concentration of the SiC in the gas phase are formed at least at one of a crystallization front of the SiC seed crystal and of the SiC single crystal which has already grown on.
In accordance with another feature of the invention, there is the step of controlling the heat flux, which conveys the SiC in the gas phase to the SiC seed crystal, by use of the insert made from the glassy carbon in a form of a hollow cylindrical gas duct.
In accordance with a further feature of the invention, there is the step of setting the heat flux with a principal direction which is oriented virtually parallel to a first center axis associated with the SiC single crystal to be produced.
In accordance with another added mode of the invention, there is the step of controlling a further heat flux, which leads to dissipation of heat at the SiC seed crystal, by a plate formed from the glassy carbon and disposed between the SiC seed crystal and a wall of the crucible.
In accordance with a concomitant feature of the invention, there is the step of controlling an additional heat flux, which leads to thermal insulation of the stock, using a further plate made from the glassy carbon and disposed between the stock and a further wall of the crucible.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a device and a method for producing at least one SiC single crystal, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.